More specifically, the invention relates to a level meter suitable for continuously measuring the quantity of gas or biogas contained in said gasometers or gasholders.
According to known art, membrane gasometers consist of a closed shell made of an airtight membrane.
An improved technique is used in the manufacture of pressostatic-type membrane gasometers, which comprise a first inner membrane (gas membrane), which delimits a gas accumulation chamber, and a second more external membrane (air membrane), arranged for creating a pressurisation chamber between the two membranes, generally filled with air, adjacent to said gas accumulation chamber.
The accumulation chamber is connected to inlet and outlet pipes for the gas contained therein, or it communicates directly with a tank below for the storage of liquids or sludges, from which said gas emanates.
The pressurisation chamber, on the other hand, is connected to an auxiliary air ventilator or to a compressor, enabling a given pressure to be maintained inside the chamber.
In a common type of pressostatic gasholders, the first membrane forms a dome above a base area, and the second membrane forms an outer dome that encloses the first one.
Said base area can be a concrete base, for example, lined with another membrane attached to the first membrane along the edge, or the same surface of a liquid in which the edge of the first membrane is immersed.
Most traditional membrane gasometers generally have a hemispherical dome, spherical cap, “three quarter sphere” or cylindrical dome shape, the latter having a base area with a substantially rectangular or elliptical shape.
Membranes are flexible, generally made of a fabric of polyester fibres spread over a plastic material, for example PVC, and must be well anchored to the ground or to a specific fixed structure.
For the correct use of said gasometers, it is necessary to know the extent of filling of the gas accumulation chamber, both for the management and regulation of the gasholder, during normal operation, and for safety purposes. Since the second membrane encloses the first externally, this is not possible with a simple visual inspection, and requires the use of a specific measuring device.
According to known art, there are two different types of measuring devices for this purpose: a first type comprises instruments that can be defined as “distance measurement” devices, while a second type comprises instruments that can be defined as “force measurement” devices.
The first type of instrument enables to measure the distance between the top of the first membrane and the corresponding top of the second membrane. As a matter of fact, the latter generally maintains the same form, since it is pressurised, while the first membrane rises and falls depending on the quantity of gas it contains; consequently, the measurement of the distance between the tops of the membranes gives a value that can be easily correlated geometrically to the quantity of gas contained in the accumulation chamber.
To this aim, patent application FR 2 766 255 describes the use of an echo instrument, generally an ultrasonic probe, associated with the outer membrane, arranged for emitting wave impulses in the direction of the inner membrane. Said impulses strike the inner membrane and are reflected and re-directed back towards the emitting instrument: the time taken for the waves to go and return is processed and converted into an electrical signal proportional to the measurement of the distance between the two membranes, and then correlated to a value of the volume of gas contained in the accumulation chamber, expressed as a percentage or as an absolute value, and displayed on a specific digital indicator panel capable of generating a voltage or current signal, generally 0-12 volts or 4-20 mA. This echo instrument is often used with a flat metallic disc, a sort of target, placed on the top of the inner membrane, and intended as a safe reflective surface for the ultrasonic waves.
A second “distance measurement” instrument is described in patent EP 0 333 698, which describes a system comprising a reel stably fixed to the top of the outer membrane of the gasometer, from which a wire unwinds, the end thereof being fixed to the top of the inner membrane. The reel is provided with a torsional spring arranged for rewinding the wire itself. As the wire unwinds or rewinds, following the movements of the inner membrane as it rises or falls as the gas content varies, a potentiometer records the rotation of the reel and thus gives a measurement of the distance between the tops of the membranes, which can also be correlated to the volume of gas contained in the accumulation chamber and displayable on a suitable digital indicator panel capable of generating a suitable electrical signal.
This first type of device for measuring the quantity of gas contained in the accumulation chamber of gasometers has certain disadvantages, common to both applications described above.
The gas membrane, since it is made of a non-stretch, non-elastic cloth, when the accumulation chamber empties, collapses and falls in on itself, forming folds and loops. In addition, the shape assumed by the accumulation chamber during each gas emission cycle is different: this leads to a non bijection between the values recorded by the measurement instruments and the actual volume of gas present in the chamber.
This disadvantage is accentuated if the gasometer is not a hemispherical dome or spherical cap type gasometer. In particular, if the gasometer comprises a ¾ sphere dome, the gas membrane, when the accumulation chamber empties, as well as collapsing and creating loops, may become unbalanced and fall outside its projected area, leading to even more imprecise measurements.
Furthermore, the weight concentrated on the first membrane of the flat metallic disc used in association with the ultrasonic probe has a negative influence on the shape of the gas accumulation chamber, thus causing unbalance of the same membrane when the chamber empties: the membrane tends to fall in an unbalanced, uncentered manner with respect to the projected area of the dome, thus leading to an imprecise volume reading subject to casual errors that are different for each cycle and therefore cannot be standardised. In an attempt to remedy this problem, the cited patent suggests the use of connecting cables between the first and second membranes, arranged regularly along the maximum circumference of the spherical surface. Said solution, although preventing the loss of balance, disadvantageously distorts the volume reading: when the disc touches the surface on which the gasometer rests, the gasometer is not empty, but inside the accumulation chamber, in the remaining semi-toroidal shape area that forms around the disc, there remains a considerable quantity of gas. Inside the gasometer, therefore, the volumetric value recorded and used for the storage capacity is considerably lower than the actual rated geometric volume.
The main disadvantage of using a cable fixed to the top of the inner membrane is that the continuous cycles of winding and unwinding the cable tend, over time, to damage the torsional spring used to rewind the cable and, consequently, the device can no longer ensure precise readings of the recorded value. Lastly, the weight of the gas membrane is always greater than the cable retraction force provided by the elasticity of the torsional spring in the reel: this leads to the inevitable unbalancing of the membrane when the accumulation chamber empties, with the creation of a peripheral volume of gas that cannot be detected.
As previously mentioned, there exists a second type of instrument for measuring the quantity of gas accumulated in gasometers, which can be defined as “force measurement” devices, which comprise means of measuring the weight of a flexible element hooked onto the outer membrane of the chamber of pressurisation air and supported by the gas membrane. These instruments correlate a dynamometric measurement of the weight force to the quantity of gas contained.
Patent application EP 1 338 843 from the same applicant describes the use of a measurement instrument of such a type, comprising a chain, one end thereof hanging from a load cell type device applied to the top of the outer membrane, while the other end is free, and simply rests on the top of the gas membrane. A raised edge is placed on the top of said gas membrane, arranged for delimiting a narrow zone on which said chain rests and in which it is contained.
The load cell measures the overall weight of the chain that is not supported by the gas membrane and provides an electrical signal. When the accumulation chamber is partially or completely filled with gas, a part of the chain rests on the top of said chamber and the load cell measures the traction due only to the weight of the part of the chain that remains suspended. In this way, the electrical signal generated by the load cell provides an indirect measurement of the height of the top of the accumulation chamber.
Advantageously, this measurement can be easily correlated to the quantity of gas stored, besides the raised edge keeps the chain resting on the central area of the top of the accumulation chamber, preventing the chain from sliding sideways, which could compromise the precision of the measurement.
However, for the measurement of the quantity of gas present, this second solution also has several disadvantages.
The chain also constitutes, although to a lesser extent than the metallic disc used in association with the ultrasonic probe, a load on the central portion of the inner membrane, which is thus still subject to the risk of becoming unbalanced and falling in an uncentered manner with respect to the projected area of the dome of the gasometer and, disadvantageously, there remains the risk of erroneous and imprecise measurements by the load cell due to the effect of the residual volume remaining in the peripheral area when the central zone of the membrane has already reached the base plane, corresponding to the maximum distance away from the top of the outer membrane.